The present invention is directed to methods and apparatus for the production of high temperature gaseous plasmas. In particular, the present invention is directed to methods and apparatus for the production of plasmas for the purpose of producing ion beams.
Plasmas are created when a gas is excited to a sufficiently high energy level to ionize at least a portion of the atomic or molecular species constituting the gas, producing a high temperature collection of free electrons, positively ionized gaseous species, and neutral, non-ionized gaseous species. For various practical reasons plasmas are typically produced in low pressure plasma chambers. Due to their combination of high temperature, low pressure, and the presence of free electrons and electrically charged gaseous ions, plasmas are characterized by unique physical and electrical properties that make them useful for various purposes. For example, plasmas are used to etch, clean and otherwise modify the surfaces of solid articles, by exposure of an article to a plasma maintained in a plasma chamber.
In particular, plasmas are used to generate positively charged ion beams, by extraction of positive ions from a plasma and their acceleration through an electric field. Depending on the elemental composition of the gas used to generate the plasma, beams ranging from proton beams to beams of heavy metal ions can be produced. Such beams are useful in applications including the ion beam etching of semiconductors, the surface treatment of solid articles by ion implantation, and emerging lithographic techniques.
The phenomenon known as electron cyclotron resonance has been used for some years for the purpose of producing plasmas. Electron cyclotron resonance (ECR) occurs when free electrons in a magnetic field are subjected to a radio frequency (RF) electrical field. ECR is possible because free electrons, when in the presence a magnetic field, tend to circulate around the axis of the magnetic field at a specific frequency, which is known as the cyclotron frequency and which is characteristic of the fundamental physical properties of the electron. When an RF signal is applied to such electrons, they can be accelerated to high energies in a resonance condition, while continuing to be constrained to orbital paths around the flux lines of the magnetic field. Under appropriate resonance conditions, electrons having energies of several million electron volts (MeV) can be produced.
More importantly, if neutral gaseous species are present under such ECR conditions, collisions between the energetic electrons and the neutral gaseous species result in ionization of the gaseous species, producing positive ions as well as additional free electrons, and generating a plasma of excited electrons and positively charged ions. The neutral gaseous species may be either monoatomic gaseous species or molecular gaseous species. With sufficiently energetic electrons, multiple electrons may be stripped from atoms of higher atomic number to produce atomic ions having high positive charges.
Under appropriate conditions, the positively charged ions so generated in a plasma can be accelerated through a static electric field to produce a beam of positive ions. By continuously applying an RF power signal to the plasma, while also replenishing the gaseous species in the plasma, a continuous ion beam can be generated in this manner. Since the plasma is typically maintained only in the region where the magnetic field exists, an ion beam extracted from the region of plasma generation is useful as an isolated, directed beam of ions outside the plasma
U.S. Pat. No. 4,417,178 to Geller, issued on Nov. 22, 1983, discloses an ECR ion beam source as described above. Much subsequent research has been directed to the further development of ECR-based ion beam sources of this kind. For example, considerable effort has been directed to the shaping of the magnetic field so as to contain the ionized species in the plasma chamber for as long a time as possible prior to their extraction, to thereby allow maximum ionization and thus maximize the positive charge of the ionized species that are extracted from the plasma chamber through a beam port. For example, a xe2x80x9cdouble humpedxe2x80x9d magnetic field, i.e. a magnetic field having a pair of peak field strengths spaced along a common field axis, is known to increase the retention of ionic species and thus result in generation of more highly charged ion species. Under appropriate conditions as much as 70% of the gas injected into such a plasma chamber can be ionized through the use of such improvements in ECR-based ion beam generators.
Because of their rather different applications and designs, ECR ion beam generators are typically categorized as either a source of highly charged heavy ions (typically for nuclear physics applications), or as a source of higher current ions having low positive charges (+1 or+2) for ion beam accelerators or for materials processing applications. Production of hydrogen ions tends to dominate accelerator applications while sources of boron, nitrogen, carbon, oxygen, and argon ions are widely used in materials processing such as ion implantation or ion etching. ECR-based plasma generators are also used for the purposes noted above for plasmas in general, such as plasma etching and cleaning. In some applications, the low charge-state ion beam sources serve as plasma injectors for the heavy ion beam generators.
As the various applications of EIR ion beam generators have expanded, it has been sought to increase their efficiency and to reduce their size and electrical power consumption. One obstacle to the reduction in size of ECR ion beam generators has arisen from the physical characteristics of the waveguides typically used to transmit RF signals to the plasma chamber, as well as the necessity of matching the impedance of the waveguide with the impedance of the plasma chamber in order to avoid loss of RF power by reflection at the interface.
Accordingly, it is the object and purpose of the present invention to provide an improved plasma generator. More particularly, it is an object and purpose of the present invention to provide a plasma generator having both smaller physical dimensions and lower electrical power requirements, as compared with previously known plasma generators.
It is also an object and purpose of the present invention to provide an ion beam source based on such an improved plasma generator.
In particular, it is an object and purpose of the present invention to provide an improved ECR-based ion beam source having reduced size and lower power consumption relative to comparable ECR-based sources known in the prior art.
The present invention provides a plasma generator useful for the production ion beams as well as for other purposes. The plasma generator includes a plasma chamber body formed of a dielectric material and having an interior cavity which functions as a plasma chamber. A solenoidal magnet surrounds the plasma chamber body and thereby maintains a magnetic field in the cavity. The plasma chamber body is coupled to a radio frequency waveguide for introducing a radio frequency signal into the plasma chamber body. The plasma chamber body is composed of a dielectric material such as boron nitride. Because the plasma chamber body surrounds the plasma chamber and also functions as an extension of the waveguide conveying the RF signal to the plasma chamber, the application of the RF signal to the plasma is enhanced and the efficiency of plasma generation and maintenance through the phenomenon of electron cyclotron resonance is improved.
Preferably both the waveguide and the plasma chamber body are composed of a dielectric material such as boron nitride, thereby allowing for a reduction in size of both the chamber and the waveguide, while also matching the impedance of the chamber with the impedance of the waveguide so as to minimize loss of RF power by reflection at the interface.
With an appropriate beam port and apparatus for generating an electric field outside the plasma chamber, ions can be extracted from the plasma and accelerated, so as to enable the plasma generator to be readily utilized as an ion beam generator that is characterized by smaller size and lower power consumption than comparable ion beam generators not having the dielectric chamber body or the dielectric waveguide.
Further improvements in efficiency can be achieved by the use of an RF signal having a particular circular polarization, selected on the basis of the direction of the magnetic field of the solenoidal magnet, so as to optimize coupling of the RF signal to the plasma.
These and other features of the present invention will be more apparent upon consideration of the accompanying drawings and the following detailed description of a preferred embodiment.